As described in the U.S. Pat. No. 5,605,118, a modern steam generator can include a complex configuration of various thermal and hydraulic units for preheating and evaporating water and generating superheated steam. Such units are typically designed to ensure complete and efficient fuel combustion while minimizing emissions of particulate and gaseous pollutants, steam generation at a desired pressure, temperature and flow rate; and maximize recovery of the heat produced upon combustion of a fuel.
Steam generators typically form part of steam plants that further include a series of steam turbines that extract work from steam from the steam generator and a condensate return system in which condensed steam is returned to the steam generator. As described in PCT patent application 2011/057881 A1 steam may be extract from an intermediate stage of the last steam turbine of the series and used-to pre heat condensate before it enters the enters a steam generator. As discussed in PCT application no. 2011/141942 A1, intermediate stage extraction may also be used to regenerate a working fluid in organic Rankine Cycles.
Reheaters and superheaters of a modern steam generator typically have specially designed tube bundles that are capable of increasing the temperature of saturated steam to specific steam outlet temperatures, while ensuring metal temperatures do not become too hot and steam flow pressure losses are minimised. Essentially, these reheaters and superheaters are single-phase heat exchangers comprising tubes through which steam flows, and across which the combustion or flue gas passes. Typically, reheater and superheater tube bundles are made of high temperature steel alloys.
The reheater typically provides steam for a second steam turbine that fluidly follows a first steam turbine that typically is fed directly from a feed water cycle that passes through the steam generator. Referring to the respective state of expansion, the first steam turbine is typically known as high-pressure or HP steam turbine and the second steam turbine or steam turbine group as the intermediate pressure or IP steam turbine/steam turbine group.
For carbonaceous fuel boiler-turbine power plants, it can be important for the heat rate and cycle efficiency to regulate and control reheater steam temperature within narrow limits to ensure that hot reheat temperature is kept close to nominal levels. This can be particularly challenging when a power plant operates at low load, for example during start-up when the pressure of the reheat section is very low. Depending of the type of steam generator or boiler, under such conditions, the reheater outlet temperature (RHO) required at main continuous rated (MCR) conditions may not be achieved. As a consequence, the IP steam turbine will not receive steam heated to the optimal operating temperature thus requiring control measures to be implemented.
In designs where the reheater surface is maintained in a condition conducive to convective heat transfer, a known method for controlling reheater temperature involves increasing or reducing the flue gases flowing over heater sections thus utilising variations in thee convective heat transfer coefficient. This method is most often used in wall fired units where the second pass of the boiler is divided in to two parallel paths up to the economizer and reheater. Typically, such designs ensure that a one third two third ratio of flow area between the low temperature superheater and the reheater is achieved. For such arrangements, dampers may be located at the bottom of flue gas passages where they, may be used to optimise flue gas flow. Advantageously the dampers may be located in the bigger flow area so that closing of the dampers will divert flue gas to the smaller flow area where the reheater surface is located. This increases the pickup in the reheater steam and thus increases the outlet temperature of the reheater. Alternatively, reducing the flow by opening the damper in the other parallel path will reduce the flue gas flow through the reheater section and thus reduce the reheater steam outlet temperature. Even though the logic of this design is simple, the use of such systems in coal and low grade fuels systems can cause construction and maintenance challenges.
Another method of controlling reheat steam temperature involves shifting the burner flame in the boiler. This is particularly applicable for tangential fired boilers. In this method burners are located in corners and tilted up or down in unison to increase radiant heat going to the reheater surface, thus affecting the superheater heat absorption. The burner tilting mechanism is so designed that all the burners in all corners tilt up or down based on the reheater outlet steam temperature. It has been the experience of some operators using low grade coal that if burners are not regulated moved, the tilting mechanism has a tendency to seize. A second problem with this method is that during low load operation, the effect of burning tilting may not be enough to prevent the hot reheat temperature dropping off more than the live steam temperature.
German patent application no. 44 47 044 C1 discloses another method of adjusting reheat temperature that involves extracting upstream of a first high pressure steam turbine and adding this extracted steam to exhaust steam of the high pressure steam turbine before the exhaust steam is reheated.
Other alternate methods for reducing reheat steam temperature also exist. For example, water sprays, also called direct contact attemperation or de-superheating, can be introduced into the fluid entering the reheater. One problem with this solution is that it can have a negative effect on cycle efficiency. Another method is to use excess air supplied to the boiler to control reheater steam temperature. This method can also have a negative effect on boiler efficiency. Further solutions include drawing off steam from the super heater and/or reheater, leaving, however, the problem of finding a disposal path for the extracted steam. An additional disadvantage of all these alternate methods is that they can only be used to reduce reheat temperature and therefore are not effective when the reheat temperature needs to be preferably increased.
In view of the prior art it is seen as an object of the present invention to provide more efficient means and methods for controlling the temperature of the reheater, particularly at low (i.e sub-operational) loads or pressure in the steam path.